Where: Rio Arriba County, New Mexico (36.5° N, 107.0° W: paleocoordinates 36.9° N, 105.3° W)
• coordinate based on political unit
When: upper tuffaceous zone Member (Chamita Formation), Hemphillian (10.3 - 4.9 Ma)
Primary reference: B. J. MacFadden. 1977. Magnetic polarity stratigraphy of the Chamita Formation stratotype (Mio-Pliocene) of North-central New Mexico. American Journal of Science 200 [J. Alroy/J. Alroy/J. Ju]more details
PaleoDB collection 19533: authorized by John Alroy, entered by John Alroy on 27.09.1993
Creative Commons license: CC BY (attribution)
• same level as Osbornoceros Quarry
upper tuffaceous zone dated by McIntosh and Quade 1995 using SCLF Ar/Ar on sanidine at between 6.93 +/- 0.05 Ma (base) and 6.78 +/- 0.03 Ma (top); this replaces older FT date on zircon of 5.6 +/- 0.9 Ma for an ash 35 m above the quarries; paleomag correlation is said by MacFadden to be with lowest reversed in the Gilbert, but this matches the banding pattern and new dates poorly, and both McIntosh and Quade 1995 and Lindsay et al. 1984 correlate it with upper "VI" reversed, i.e., 3Ar of Cande and Kent 1992
dates of Dethier et al. 1986 for the Chamita Fm. are for the base of the Fm.
Gomphotherium sp. and Hemiauchenia cf. vera from "6.93" Ma level of Latorre et al. 1997 may pertain to these quarries
Pliometanastes sp. Hirschfeld and Webb 1968: "Santa Clara Canyon" stratigraphically between San Juan and Leyden quarries (relative order not indicated)
"Vulpes stenognathus" is listed by MacFadden et al. 1979 but not discussed; this could be a typographical error
"Osbornoceros osborni" appears to be from the Osbornoceros Quarry of Tedford and Barghoorn 1993 (see); however, Frick 1937 lists this species from "W. Rio Grande, above San Juan"
"Ilingoceros alexandrae" is apparently the same record said by Tedford and Barghoorn 1993 to be from an indeterminate level in the Fm.
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